1. Field of the Invention
The invention relates to a method for removing harmful substances, in particular dioxin, from flue gas generated in for example a waste incinerator, using an adsorption agent.
2. Description of the Prior Art
In the waste processing plants that have been used so far domestic waste is processed by means of incineration. The term domestic waste is not only understood to mean waste from private individuals, but also waste from companies which is similar to the waste produced by private individuals as regards its composition, this in contrast to industrial waste.
When domestic waste is incinerated energy is released in the form of heat. The energy that is released is used either for generating electricity or for district heating. The oxygen required for the incineration of domestic waste is supplied with ambient air. The principal residual products of the incineration process are slag, fly ash, flue gas scrubbing residue and flue gas.
In order to prevent the harmful substances that are released during the incineration process from finding their way into the environment via the flue gas, the waste processing plants are provided with a flue gas scrubbing plant. Said harmful substances include: hydrocarbons (CxHy), cadmium (Cd), carbon monoxide (CO), chloric acid (HCl), hydrogen fluoride (HF), mercury (Hg) (plus other heavy metals), dioxin, sulphur dioxide (SO.sub.2), nitrogen oxides (NO.sub.x) and fly ash.
Generally the term dioxin is understood to mean polychlorodibenzoparadioxin (PCDD) and polychlorodibenzofuran (PCDF). These two substances have the following structural formula: ##STR1##
A method for removing dioxin from flue gas from a waste processing plant is known from European patent No. 0 574 705 in the name of Sumitomo Heavy Industries Ltd. With this known method the removing of dioxin takes place by carrying the flue gas through a so-called "moving bed absorber of the cross-flow type" at a temperature of 90-120.degree. C. Active carbon or active coke is thereby supplied to the above absorber, which carbon or coke is to make contact with the flue gas that is likewise supplied thereto. Then said active carbon and the dioxin adsorbed therein is regenerated by being heated and subsequently returned to said absorber.
One drawback of the known method is that it is unattractive from an economic point of view because an additional adsorption device (inclusive of regeneration agents) has to be used besides the actual waste processing plant. In addition to that the active carbon must be processed separately after having been regenerated a number of times. Because of this the known method is not only costly, but also complex. In practice it has become apparent, therefore, that although dioxin is removed from the flue gas by using the method according to the aforesaid European patent is applied, this takes place in a costly and inefficient manner.
The object of the invention is to provide an economical and efficient method for removing harmful substances, in particular dioxin, from flue gas, and in order to accomplish that objective a method of the kind referred to in the introduction is characterized in that after the flue gas has been washed with a washing medium, the adsorption agent is discharged together with said washing medium for subsequent collection of the adsorption agent, whereby the magnitude of at least one parameter which (partially) governs the dioxin content of the flue gas/washing medium is adapted to the (degree of) adsorption or desorption, as the case may be, of dioxin by the apparatus used for carrying out the method. More in particular said parameter is selected from the group of amount of adsorption agent, amount of waste incinerated per time unit, furnace temperature and amount of air supplied for the incineration of waste. As a result of this the method according to the invention is extremely simple and elegant, because no separate adsorber is required. The invention is in particular based on the perception that a controlled low dioxin emission is only achieved when the effect of adsorption or desorption of dioxin by (the lining of) the apparatus being used (which has not been recognized so far) is taken into account. By increasing or decreasing the amount of adsorption agent in the case of adsorption or desorption respectively a condition of equilibrium is created between the adsorption of dioxin and the release thereof by the (lining) of the apparatus, and that in a controlled manner. Preferably the adsorption agent is injected, more in particular in the form of a powder, before the flue gas is washed. This makes it possible for the adsorption agent to come into intimate contact with the flue gas in a very efficient manner in a relatively short period of time, as a result of which a practically complete adsorption of the dioxin will take place. Said intimate contact is effected by the turbulence of the flue gas. Extensive testing has surprisingly shown that in this manner the bulk of the dioxin is removed from the flue gas. It is noted in this connection that according to the invention a limit value for the emission of dioxin of less than 0.4 g TEQ/m.sup.3, in particular less than 0.1 ng TEQ/m.sup.3, is achieved.
One preferred embodiment of a method according to the invention is characterized in that the dioxin content of the flue gas is measured before and after the removal of dioxin therefrom (in particular before and after the washing of the flue gas), as well as the dioxin content of the washing medium being discharged, on the basis of which measurements the dioxin content of the washing medium resulting from the adsorption or desorption by the apparatus used in carrying out the method is calculated, whereby the magnitude of the parameter is controlled in dependence on the (degree of) said adsorption or desorption.
Another preferred embodiment of a method according to the invention is characterized in that said adsorption agent is injected after a first cooling down of the flue gas to a temperature below 400.degree. C., preferably below 300.degree. C., in particular a temperature of 250.degree. C. or below. Extensive testing has shown that the formation of dioxin predominantly takes place in the above temperature ranges.
The most efficient method is, therefore, to remove the dioxin from the flue gas immediately after the formation thereof by adsorbing it to the adsorption agent.
Another preferred embodiment of a method according to the invention is characterized in that the adsorption agent is injected while the flue gas is being washed. Due to the intimate contact of the liquid with the dioxin during the washing of the flue gas a considerable amount of dioxin is adsorbed.
Another preferred embodiment of a method according to the invention is characterized in that the adsorption agent is injected during a first washing stage in an at least substantially acidic environment. An important advantage of this is that other harmful substances, such as heavy metals, are adsorbed, and thus removed from the flue gas (which is wet as a result of being washed, and isolated in the washing medium (washing water)).
Another preferred embodiment according to the invention is characterized in that the adsorption agent is injected during a second washing stage (while adding a neutralisation agent, preferably caustic soda) in an environment which is at least substantially less acidic than that of the first washing stage. It is preferred to inject the adsorption agent also in a third washing stage, whereby use is made of a venturi. In particular any heavy metals and dioxin that remain (often in the rubber lining of the washers used in the various washing stages) will thereby adhere to the adsorption agent.
Another preferred embodiment of a method according to the invention is characterized in that at least one of the substances from the group of active carbon, brown coal, coke, lime, lava rock and pumice stone is used as the adsorption agent.
The invention also relates to a device for carrying out the method.